A thermographic infrared camera typically uses a thermographic infrared sensor to capture an infrared image. Localized changes in temperature caused by infrared irradiation are detected by the thermographic infrared sensor. The sensor detects localized changes in temperature through changes in a value of a physical property of the sensor, such as localized changes in electrical resistance, electromotive force, or electrical charge.
A conventional thermographic infrared sensor typically has an inferior sensor sensitivity compared to a quantum infrared sensor. However, in contrast to quantum infrared sensors, thermographic infrared sensors do not require a coolant supply (e.g., a supply of liquid nitrogen) for operation. Consequently, the size of an infrared camera comprising a thermographic infrared sensor can be reduced which is advantageous for the various current uses of infrared cameras, such as crime prevention cameras or emergency monitoring cameras.
Conventional thermographic infrared sensors include multiple thermosensing elements, such as bolometers, each exhibiting an electrical resistance that varies with a change in temperature caused by infrared irradiation. Utilizing recently developed microfabrication techniques, several hundred bolometers may be formed in an array on a single semiconductor substrate. An air gap is typically formed between the substrate and each bolometer to prevent heat dissipation from the bolometer to the substrate. Such air gaps improve the bolometers' absorption efficiency of infrared irradiation.
FIG. 8 shows a conventional thermographic infrared camera which comprises a photographic lens 51 positioned to transmit infrared irradiation. An infrared image sensor 52 is positioned to receive infrared irradiation from the photographic lens 51. The infrared image sensor 52 includes an array of bolometers and is enclosed within a vacuum container 54 together with a thermoelectric cooler (TE cooler) 53. The TE cooler 53, typically comprising a Peltier element, maintains a constant temperature inside the vacuum container 54 to prevent the bolometers from being affected by temperature changes in the surrounding environment.
As a bolometer's temperature changes, its electrical resistance also changes which changes the electrical current that can flow through the bolometer at a given voltage. A scanning circuit is provided in the infrared image sensor 52 to sequentially apply a bias voltage to each bolometer. The bias voltage is applied in synchrony with a start pulse and a clock pulse produced by a drive pulse generation circuit 56. The scanning circuit then detects the electrical current level of each bolometer.
In prior-art thermographic infrared cameras, the detected electrical current through each bolometer is converted to a corresponding voltage by a suitable current-voltage converter. The voltage is then converted to a corresponding digital signal by an analog-to-digital (A/D) converter 57. An offset correction circuit 58 corrects any offset based on offset data stored in an offset data random-access memory (RAM) 59. A gain correction circuit 60 is typically utilized to correct gain, based on correction data stored in a gain correction data read-only memory (ROM) 61. The signals corrected by both correction circuits 58, 60 are then converted to corresponding video signals by a digital-to-analog (D/A) converter 62 and a video signal generation circuit 63. The video signals are displayed as infrared images on a monitor 64.
Because the resistance of each bolometer changes in proportion to a change in temperature of the bolometer caused by infrared irradiation, the rate of change K.sub.t of the bolometer conductance per degree Celcius may be expressed by Equation (1) as follows: EQU K.sub.t =(1 /.DELTA.T) (.DELTA.G/G) (1)
wherein G is the bolometer conductance, .DELTA.T is the temperature change, and .DELTA.G is the change in conductance G when the temperature varies by .DELTA.T.
Whenever a bias voltage V.sub.b is applied to the bolometer, a bias current output I.sub.b may be expressed by Equation (2) as follows: EQU I.sub.b =G.multidot.V.sub.b (2)
With a change in conductance .DELTA.G (due to the localized absorption of infrared irradiation), the electrical current change .DELTA.I exhibited by the bolometer may be expressed by Equation (3) as follows: EQU .DELTA.I =.DELTA.G.multidot.V.sub.b =K.sub.t .multidot..DELTA.T.multidot.G.multidot.V.sub.b (3)
Whenever no infrared irradiation is incident on the bolometer, the output signal from the bolometer is regarded as "background output." In the absence of infrared irradiation, the conductance G of each bolometer can still vary (as shown in Equation (2)), and thus, the output I.sub.b can still vary. The variation in output I.sub.b in the absence of infrared irradiation is superimposed, as an offset, on the output I.sub.b in the presence of infrared irradiation and is termed fixed pattern noise (FPN).
As shown by Equation (3), variations in the rate of change K.sub.t, temperature variations .DELTA.T, and conductance variations .DELTA.G of each bolometer influence the change in electrical current .DELTA.I, i.e., the gain of each bolometer. In a prior-art thermographic infrared camera, these variations must be corrected by an offset correction circuit 58 and a gain correction circuit 60. More specifically, in a prior-art infrared camera, to correct gain it is necessary to first determine a gain correction coefficient. The gain correction coefficient is used to cause the temperature of a reference blackbody to vary. The resulting output of the affected infrared image sensor 52 is then read into a computer that calculates the corresponding gain correction coefficient for each bolometer in order to obtain a uniform output from all the bolometers.
In such prior-art cameras, in order to determine an accurate gain correction coefficient, the temperature of the TE cooler 53 must be maintained with utmost accuracy. Such exacting temperature control requirements result in relatively expensive and complex infrared cameras.
Additionally, in certain prior-art infrared cameras, in order to accurately detect localized temperature variations in the sensor due to infrared irradiation, any temperature variations that could be transmitted to the sensor from the external environment had to be eliminated. To address this problem a reference bolometer is included inside the infrared image sensor 52. Adverse effects of variations in the surrounding environmental temperature can be eliminated by minimizing the temperature difference between the bolometers in the array and the reference bolometer. However, as the environmental temperature increases (with a concomitant increase in the temperature of the bolometers in the array), the output level of each bolometer in the array fluctuates more due to a greater variation in the conductance of each element that occurs with increased temperature. Therefore, in prior-art infrared cameras, a constant temperature inside the vacuum container 54 must be maintained, adding to the cost and complexity of the camera.